1. Field of the Invention
The present invention relates to a development unit, for use in an electrophotographic copier or a printer, which includes a plurality of developers containing toner of different colors and visualizes an electrostatic latent image formed on an image carrier, such as a photosensitive drum, by means of a developer arbitrarily selected from the developers. More particularly, the present invention relates to an improvement in a so-called rotary development unit which exchanges a developer with another one in a position opposite to the image carrier while rotating the developer around a rotary shaft.
2. Description of the Related Art
An existing color image forming apparatus which employs an electrophotographic method includes a well-known color image forming apparatus which employs a so-called multiple transfer method. In this apparatus, yellow, cyan, magenta, and black toner images are sequentially transferred to a recording sheet, and these toner images are superimposed on each other to thereby form a full color image.
In the multiple transfer method, it is necessary to form a toner image of each color on an image carrier, such as a photosensitive drum, one after another. For this reason, a plurality of developers, each of which contains toner of each color, are disposed in a position where any one color developer is selectively placed opposite the image carrier. The other developers must be held in a receded position separated from the image carrier.
To this end, an existing color image forming apparatus employs a so-called rotary development unit (A-Color manufactured by Fuji Xerox Co., Ltd.) as means for selectively setting developers of four colors with in a development position. In this rotary development unit, a plurality of developers are positioned at equal intervals around a cylindrical rotary member (hereinafter referred to as a developer exchanger). As a result of revolution of the developer exchanger, an arbitrary developer is set in the development position, and the other developers are held in a receded position relative to the position where the developer is in close proximity to the image carrier.
If color toner, such as yellow toner, cyan toner, or magenta toner, is mixed with magnetic powder, the color developing characteristics of the color toner are deteriorated. For this reason, the color toner is usually non-magnetic toner, and each developer of the aforementioned color image forming apparatus develops an electrostatic latent image formed on the image carrier by means of a two-component development method; i.e., a two-component developing agent including a mixture of non-magnetic toner and magnetic carriers.
According to the development method that employs the two-component developing agent, only the toner of the mixture of toner and carriers contained in the developer is consumed through development of an electrostatic latent image. Therefore, each of the developers retained by the developer exchanger should be replenished with new toner (hereinafter referred to as fresh toner). In order to constantly maintain a desired development density of an electrostatic latent image, the quantity of fresh toner to be supplied to the developer must be strictly controlled.
The applicant of the present patent application has already put forward a rotary development unit which satisfies the foregoing demand. The rotary development unit is arranged so as to permit insertion or removal of a toner supply container storing fresh toner into or from the developer exchanger in such a way as to be in proximity to each color developer. The toner supply container attached to the developer exchanger is connected to the color developer through a toner supply channel disposed in parallel with the rotary shaft of the development exchanger. A conveyor screw comprising a rotary shaft around which a helical vane is wrapped (hereinafter referred to as an auger) is provided so as to pass through the toner supply channel. The fresh toner unloaded to the toner supply channel from the toner supply container is fed to the developer by rotation of the auger.
In the rotary development unit having the foregoing structure, the quantity of fresh toner corresponding to the number of rotations of the auger disposed in the toner supply channel is fed to the developer. Accordingly, if toner consumption is predicted from the image data read by the image forming apparatus, and the number of rotations of the auger is adjusted so as to correspond to the predicted toner consumption, the development density of an electrostatic latent image can be constantly maintained.
In the previously-described rotary development unit, the developers, the toner supply containers, and the toner supply channels revolve around the rotary shaft of the developer exchanger every time the developer exchanger is rotated for the purpose of exchanging the developer to be placed in the development position. Even in a state in which the augers housed in the toner supply channels are stopped, fresh toner moves along the helical vane of the auger and is naturally conveyed through the toner supply channel.
For this reason, in a case where the developer exchanger rotates in one direction, the fresh toner held in the toner supply channel is conveyed through the inside of the toner supply channel toward the developer or the toner supply container depending on the direction in which the helical vane of the auger is wrapped. In the former case, the developer is unintentionally replenished with fresh toner, thereby resulting in an excess quantity of toner being stored in the developer. In contrast, in the latter case, the fresh toner recedes to the toner supply container from the toner supply channel. Accordingly, if an attempt is made to replenish the developer with the fresh toner by effecting a given number of rotations of the auger, the quantity of fresh toner corresponding to the number of rotations of the auger cannot be supplied to the developer, thereby resulting in toner deficiency in the developer.
Such a problem is particularly likely to arise in a developer other than a black developer after a copy job has been repeatedly executed several times in a black and white mode. In order to prevent the surface of the latent image carrier from being soiled by toner, a home position at which any developer is spaced away from a latent image carrier is ensured in the color image forming apparatus that uses the rotary development unit. In a copy wait state, the developer exchanger is usually set in its home position. If there are successive copy jobs in a black and white mode, the black developer is carried from the home position to the development position where it faces the latent image carrier at the beginning of each job. Further, after the completion of the job, the black developer is returned to the home position. As a result, the developer exchanger is rotated for each job, and the other color developers are not used at all during the course of the copy jobs. If the copy jobs are repeated several times in a black and white mode, the fresh toner held in the toner supply channel of each of the other color developers; i.e., the yellow developer, the cyan developer, and the magenta developer, is unintentionally carried, thereby resulting in the foregoing problems being more noticeable.
In the case where an excess arises in the quantity of fresh toner to be supplied to the developer, there is no alternative way but to wait for consumption of the toner contained in the developer through development over time until the development density of an electrostatic latent image is adjusted. In contrast, in the case where there is a shortage of fresh toner in the developer, it is sufficient to rotate the auger housed in the toner supply channel to the number of rotations corresponding to the insufficient amount of the fresh toner.